Allow keepSpecialComments to be set as a string
Merge pull request #153 from shama/keepSpecialComments

Allow keepSpecialComments value of 0 and 1 to be set as a string.
Version 1.1.5.
Fixes #155 - broken irregular CSS content.

* ["content"] is an invalid CSS but it was matched as an attribute.
* If placed inside content property value it was matched BEFORE we escape all free text.
Version 1.1.6.
Adds much faster removal of empty selectors & @media elements.

* Adds supports for removing empty nested @media elements.
Adds missing `@import` processing to our benchmark (run via `npm run bench`)